The pharmacokinetics of frusemide are influenced by age

Potential implications of DMET ontogeny on the disposition of commonly prescribed drugs in neonatal and pediatric intensive care units

Introduction: Pediatric patients, especially neonates and infants, are more susceptible to adverse drug events as compared to adults. In particular, immature small molecule drug metabolism and excretion can result in higher incidences of pediatric toxicity than adults if the pediatric dose is not adjusted.Area covered: We reviewed the top 29 small molecule drugs prescribed in neonatal and pediatric intensive care units and compiled the mechanisms of their metabolism and excretion. The ontogeny of Phase I and II drug metabolizing enzymes and transporters (DMETs), particularly relevant to these drugs, are summarized. The potential effects of DMET ontogeny on the metabolism and excretion of the top pediatric drugs were predicted. The current regulatory requirements and recommendations regarding safe and effective use of drugs in children are discussed. A few representative examples of the use of ontogeny-informed physiologically based pharmacokinetic (PBPK) models are highlighted.Expert opinion: Empirical prediction of pediatric drug dosing based on body weight or body-surface area from the adult parameters can be inaccurate because DMETs are not mature in children and the age-dependent maturation of these proteins is different. Ontogeny-informed-PBPK modeling provides a better alternative to predict the pharmacokinetics of drugs in children.

Contribution of Uptake and Efflux Transporters to Oral Pharmacokinetics of Furosemide

Furosemide is a widely used diuretic for treating excessive fluid accumulation caused by disease conditions like heart failure and liver cirrhosis. Furosemide tablet formulation exhibits variable pharmacokinetics (PK) with bioavailability ranging from 10 to almost 100%. To explain the variable absorption, we integrated the physicochemical, in vitro dissolution, permeability, distribution, and the elimination parameters of furosemide in a physiologically-based pharmacokinetic (PBPK) model. Although the intravenous PBPK model reasonably described the observed in vivo PK data, the reported low passive permeability failed to capture the observed data after oral administration. To mechanistically justify this discrepancy, we hypothesized that transporter-mediated uptake contributes to the oral absorption of furosemide in conjunction with passive permeability. Our in vitro results confirmed that furosemide is a substrate of intestinal breast cancer resistance protein (BCRP), multidrug resistance-associated protein 4 (MRP4), and organic anion transporting polypeptide 2B1 (OATP2B1), but it is not a substrate of P-glycoprotein (P-gp) and MRP2. We then estimated the net transporter-mediated intestinal uptake and integrated it into the PBPK model under both fasting and fed conditions. Our in vitro data and PBPK model suggest that the absorption of furosemide is permeability-limited, and OATP2B1 and MRP4 are important for its permeability across intestinal membrane. Further, as furosemide has been proposed as a probe substrate of renal organic anion transporters (OATs) for assessing clinical drug-drug interactions (DDIs) during drug development, the confounding effects of intestinal transporters identified in this study on furosemide PK should be considered in the clinical transporter DDI studies.

A Clinical Drug-Drug Interaction Study Assessing a Novel Drug Transporter Phenotyping Cocktail With Adefovir, Sitagliptin, Metformin, Pitavastatin, and Digoxin

A new probe drug cocktail containing substrates of important drug transporters was tested for mutual interactions in a clinical trial. The cocktail consisted of (predominant transporter; primary phenotyping metric): 10 mg adefovir-dipivoxil (OAT1; renal clearance (CL_R )), 100 mg sitagliptin (OAT3; CL_R ), 500 mg metformin (several renal transporters; CL_R ), 2 mg pitavastatin (OATP1B1; clearance/F), and 0.5 mg digoxin (intestinal P-gp, renal P-gp, and OATP4C1; peak plasma concentration (C_max ) and CL_R ). Using a randomized six-period, open change-over design, single oral doses were administrated either concomitantly or separately to 24 healthy male and female volunteers. Phenotyping metrics were evaluated by noncompartmental analysis and compared between periods by the standard average bioequivalence approach (boundaries for ratios 0.80-1.25). Primary metrics supported the absence of relevant interactions, whereas secondary metrics suggested that mainly adefovir was a victim of minor drug-drug interactions (DDIs). All drugs were well tolerated. This cocktail may be another useful tool to assess transporter-based DDIs in vivo.

Development of a Whole-Body Physiologically Based Pharmacokinetic Approach to Assess the Pharmacokinetics of Drugs in Elderly Individuals

Background

Because of the vulnerability and frailty of elderly adults, clinical drug development has traditionally been biased towards young and middle-aged adults. Recent efforts have begun to incorporate data from paediatric investigations. Nevertheless, the elderly often remain underrepresented in clinical trials, even though persons aged 65 years and older receive the majority of drug prescriptions. Consequently, a knowledge gap exists with regard to pharmacokinetic (PK) and pharmacodynamic (PD) responses in elderly subjects, leaving the safety and efficacy of medicines for this population unclear.

Objectives

The goal of this study was to extend a physiologically based pharmacokinetic (PBPK) model for adults to encompass the full course of healthy aging through to the age of 100 years, to support dose selection and improve pharmacotherapy for the elderly age group.

Methods

For parameterization of the PBPK model for healthy aging individuals, the literature was scanned for anthropometric and physiological data, which were consolidated and incorporated into the PBPK software PK-Sim^®. Age-related changes that occur from 65 to 100 years of age were the main focus of this work. For a sound and continuous description of an aging human, data on anatomical and physiological changes ranging from early adulthood to old age were included. The capability of the PBPK approach to predict distribution and elimination of drugs was verified using the test compounds morphine and furosemide, administered intravenously. Both are cleared by a single elimination pathway. PK parameters for the two compounds in younger adults and elderly individuals were obtained from the literature. Matching virtual populations—with regard to age, sex, anthropometric measures and dosage—were generated. Profiles of plasma drug concentrations over time, volume of distribution at steady state (V_ss) values and elimination half-life (t_½) values from the literature were compared with those predicted by PBPK simulations for both younger adults and the elderly.

Results

For most organs, the age-dependent information gathered in the extensive literature analysis was dense. In contrast, with respect to blood flow, the literature study produced only sparse data for several tissues, and in these cases, linear regression was required to capture the entire elderly age range. On the basis of age-informed physiology, the predicted PK profiles described age-associated trends well. The root mean squared prediction error for the prediction of plasma concentrations of furosemide and morphine in the elderly were improved by 32 and 49 %, respectively, by use of age-informed physiology. The majority of the individual V_ss and t_½ values for the two model compounds, furosemide and morphine, were well predicted in the elderly population, except for long furosemide half-lifes.

Conclusion

The results of this study support the feasibility of using a knowledge-driven PBPK aging model that includes the elderly to predict PK alterations throughout the entire course of aging, and thus to optimize drug therapy in elderly individuals. These results indicate that pharmacotherapy and safety-related control of geriatric drug therapy regimens may be greatly facilitated by the information gained from PBPK predictions.

Electronic supplementary material

The online version of this article (doi:10.1007/s40262-016-0422-3) contains supplementary material, which is available to authorized users.

Analysis of the variability of the pharmacokinetics of multiple drugs in young adult and elderly subjects and its implications for acceptable daily exposures and cleaning validation limits

The elderly constitute a significant, potentially sensitive, subpopulation within the general population, which must be taken into account when performing risk assessments including determining an acceptable daily exposure (ADE) for the purpose of a cleaning validation. Known differences in the pharmacokinetics of drugs between young adults (who are typically the subjects recruited into clinical trials) and the elderly are potential contributors affecting the interindividual uncertainty factor (UF_H) component of the ADE calculation. The UF_H values were calculated for 206 drugs for young adult and elderly groups separately and combined (with the elderly assumed to be a sensitive subpopulation) from published studies where the pharmacokinetics of the young adult and elderly groups were directly compared. Based on the analysis presented here, it is recommended to use a default UF_H value of 10 for worker populations (which are assumed to be approximately equivalent to the young adult groups) where no supporting pharmacokinetic data exist, while it is recommended to use a default UF_H value of 15 for the general population, to take the elderly into consideration when calculating ADE values. The underlying reasons for the large differences between the exposures in the young adult and elderly subjects for the 10 compounds which show the greatest separation are different in almost every case, involving the OCT2 transporter, glucuronidation, hydrolysis, CYP1A2, CYP2A6, CYP2C19, CYP2D6, CYP3A4 or CYP3A5. Therefore, there is no consistent underlying mechanism which appears responsible for the largest differences in pharmacokinetic parameters between young adult and elderly subjects.

Use of diuretics in the treatment of heart failure in the elderly

Diuretics are tools of considerable therapeutic importance. First, they effectively reduce blood pressure, while at the same time decreasing the morbidity and mortality associated with hypertension. Diuretics are currently recommended as first-line therapy for the treatment of hypertension. In addition, they remain an important component of heart failure therapy, in that they improve the symptoms of congestion, which typify the more advanced stages of heart failure. This article reviews the mode of action of the various diuretic classes and the physiologic adaptations that follow; sets up the basis for their use in the treatment of volume-retaining states, particularly as applies to the elderly; and reviews diuretic-related side effects that are normally encountered.

Use of diuretics in the treatment of heart failure in the elderly

Diuretics are tools of considerable therapeutic importance. First, they effectively reduce blood pressure, while at the same time decreasing the morbidity and mortality associated with hypertension. Diuretics are currently recommended as first-line therapy for the treatment of hypertension. In addition, they remain an important component of heart failure therapy, in that they improve the symptoms of congestion, which typify the more advanced stages of heart failure. This article reviews the mode of action of the various diuretic classes and the physiologic adaptations that follow; sets up the basis for their use in the treatment of volume-retaining states, particularly as applies to the elderly; and reviews diuretic-related side effects that are normally encountered.

Pharmacokinetics in older persons

BACKGROUND

Physiologic changes and disease-related alterations in organ function occur with aging. These changes can affect drug pharmacokinetics in older persons.

OBJECTIVE

This article reviews age-related changes in pharmacokinetics and their clinical relevance.

METHODS

A PubMed search was conducted using the terms elderly and pharmacokinetics. Other reviews were also included for literature searching. The review includes literature in particular from 1990 through April 2004. Some articles from before 1990 were included to help illustrate principles of age-related pharmacokinetics.

RESULTS

There are minor changes in drug absorption with aging. The effect of aging on small-bowel transporter systems is not yet fully established. Bioavailability of highly extracted drugs often is increased with age. Transdermal absorption may be delayed, especially in the case of water-soluble compounds. Fat-soluble drugs may distribute more widely and water-soluble drugs less extensively in older persons. Hepatic drug metabolism shows wide interindividual variation, and in many cases, there is an age-related decline in elimination of metabolized drugs, particularly those eliminated by the cytochrome enzyme system. Any decrement in cytochrome enzyme metabolism appears nonselective. Synthetic conjugation metabolism is less affected by age. Pseudocapillarization of the sinusoidal endothelium in the liver, restricting oxygen diffusion, and the decline in liver size and liver blood flow may influence age-related changes in rate of hepatic metabolism. Frailty, physiological stress, and illness are important predictors of drug metabolism in older individuals. Inhibition of drug metabolism is not altered with aging, but induction is reduced in a minority of studies. Renal drug elimination typically declines with age, commensurate with the fall in creatinine clearance. Renal tubular organic acid transport may decline with age, while the function of the organic base transporter is preserved but may be less responsive to stimulation.

CONCLUSION

Changes in pharmacokinetics occur due to age-related physiologic perturbations. These changes contribute to altered dose requirements in older persons, particularly in the case of drugs eliminated by the kidney. Interindividual variation, disease, frailty, and stress may overshadow age-related changes.

Pharmacokinetic dosage guidelines for elderly subjects

Ageing is associated with a decline in drug elimination; hence, using the same doses as in younger adults may result in higher plasma drug concentrations and toxicity. Two approaches are available for dose correction to account for decreased drug elimination. One procedure is based on the extrarenal elimination fraction (Q(0)) and the age-dependent changes in creatinine clearance; the other uses the decline in total drug clearance (CL). Mean values of Q(0) and CL in young and old people are reported for many drugs in the literature and are summarised in this article. Although the pharmacokinetic techniques for dose adjustment in the elderly are useful, they provide only an average dose correction and neglect age-dependent changes in drug bio-availability, plasma protein binding, the fate of active metabolites, and altered sensitivity to drugs. To account for pharmacodynamic changes in old age, clinical and/or biochemical targets should be defined as therapeutic goals. Drugs whose effects cannot be monitored in these terms should be avoided in elderly individuals.

Cardiovascular Drug Therapy in Elderly Patients

An increasing number of elderly patients are exposed to cardiovascular drugs for the treatment of acute and/or chronic conditions. This is a result of the progressive aging of the population, a common feature in most industrialised countries, and an improvement in primary and secondary cardiovascular prevention strategies with increased survival rates. Traditionally, most elderly patients receiving cardiovascular drugs had advanced cardiac, liver and kidney disease that significantly influenced drug pharmacokinetic and pharmacodynamic parameters. Currently, however, many patients without significant organ impairment receive cardiovascular therapy for primary or early secondary prevention (i.e. increased vascular risk, asymptomatic left ventricular dysfunction, poststroke phase, type 2 diabetes mellitus), highlighting the need for a better understanding of specific age-related pharmacokinetic and pharmacodynamic effects. A systematic review has been conducted on the specific effects of aging, in the absence of major co-morbidities, on the pharmacokinetic and pharmacodynamic properties of traditional and newer cardiovascular drugs. Currently, the evidence available is poor or nonexisting for several drugs and mainly derived from very small and underpowered studies, thus limiting data interpretation. In particular, there is very little information on patients >80 years of age, thus raising important concerns about the correct use of these drugs in this constantly growing population.

Studies on the metabolic fate of M17055, a novel diuretic (6). Assessment for drug-drug interactions of M17055 in metabolism, distribution and excretion

1. The potential of M17055, a novel diuretic candidate, to affect the activities of human CYP enzymes, alter the plasma unbound fraction and compete with concomitant drugs in renal secretion as part of an assessment for drug-drug interactions in metabolism, distribution and excretion was investigated. 2. The effects of M17055 on the activities of human CYP1A2, CYP2E1, CYP2C9, CYP2C19, CYP2D6 and CYP3A4 were considered negligible at clinically relevant concentrations. 3. The majority of M17055 (99%) was bound to human plasma proteins, but it is unlikely to alter the binding of other clinically relevant drugs. 4. The renal clearance of M17055 (corrected for the plasma unbound fraction in male rats) substantially exceeded the glomerular filtration rate and was markedly reduced by treatment with probenecid, suggesting that the renal excretion of M17055 is controlled predominantly by an active secretion mechanism. 5. The results show that M17055 is unlikely to cause or undergo significant pharmacokinetic interactions with concomitant drugs in metabolism and distribution. However, when it is administered simultaneously with certain organic anions, drug-drug interactions during kidney excretion may be possible.

High-performance liquid chromatography-mass spectrometric analysis of furosemide in plasma and its use in pharmacokinetic studies

This study presents a rapid, specific and sensitive high-performance liquid chromatography-mass spectrometric (LC-MS) assay for the determination of furosemide in human plasma using diclofenac as an internal standard (IS). Both compounds were extracted from human plasma with ethyl acetate at pH 1 and were chromatographed using Shim-Pack GLC-CN column and a mobile phase consisting of acetonitrile and 20 mM ammonium acetate buffer solution pH 7, 4:1 (v/v) at a flow rate 1 ml min(-1). Furosemide and IS were detected by mass spectrometer operated in the negative single ion monitoring mode using APCI as an ionization process at m/z 329.2 and 294.1, respectively. The assay linearity of furosemide was confirmed over the range 50-2,000 ng ml(-1). Detection limit for furosemide in plasma was 10 ng ml(-1). The selected concentration range corresponds well with the plasma concentrations of furosemide for pharmacokinetic study. Intraday and interday relative standard deviations were 1.3-4.7 and 2.7-11.5%, respectively. The extraction recovery percentages of furosemide and IS from plasma were in the range 89.3-97.1%. The developed LC-MS procedure was applied for the determination of the pharmacokinetic parameters of furosemide after an oral administration of tablet formulation (40 mg) to two healthy male volunteers. The calculated parameters were in good agreement with the reported values.

Drug dosage in the elderly. Is it rational?

Pharmacotherapy in the elderly requires an understanding of the age-dependent changes in function and composition of the body. Aging is characterised by a progressive loss of functional capacities of most if not all organs, a reduction in response to receptor stimulation and homeostatic mechanisms, and a loss of water content and an increase of fat content in the body. The most important pharmacokinetic change in old age is a decrease in the excretory capacity of the kidney; in this regard, the elderly should be considered as renally insufficient patients. The decline in the rate of drug metabolism with advancing age is less marked. In addition, the volume of distribution and the oral bioavailability of drugs may be changed in the elderly compared with younger individuals. Average dosage adjustments for the aged can be derived from simple equations and mean pharmacokinetic parameters from older and younger adults. However, these average dose adjustment factors neglect the large variation in the decline in organ functions among the elderly. Individual dose adjustment factors can be obtained from the drug clearance in a particular patient, where clearance/fractional bioavailability (CL/f) may be calculated from the area under the plasma concentration-time curve (AUC) of the drug in question. Using pharmacokinetic guidelines for dose adjustments, the same plasma drug concentrations result in elderly as in younger adults. However, we are frequently confronted with pharmacodynamic changes in old age which alter the sensitivity to drugs, irrespective of changes in drug disposition. For instance, the sensitivity of the cardiovascular system to beta-adrenergic agonists and antagonists decreases in old age and the incidence of orthostatic episodes in response to drugs that lower blood pressure is increased. The CNS is especially vulnerable in the elderly; agents that affect brain function (anaesthetics, opioids, anticonvulsants, psychotropic drugs) must be used very cautiously in this age group. The increased responsiveness to drugs in the elderly renders the measurement of drug plasma concentrations an attractive method to monitor pharmacotherapy in this age group. Sensitive technology to quantitatively determine plasma drug concentrations is available. However, optimal therapeutic plasma concentrations have not been established for most drugs in the elderly. Investigations concerning drug pharmacokinetic-pharmacodynamic relationships in the aged are an important area of future work in clinical pharmacology.

Determination and pharmacokinetics of a furosemide-amiloride drug combination

The study presents an accurate and precise HPLC assay for the determination of furosemide and amiloride in human specimens. Both drugs were extracted from human plasma with ethyl acetate; furosemide was extracted at pH 1 and amiloride at pH 12. While chromatographic separation conditions, i.e., column, mobile phase and flow-rate were the same for both investigated drugs, furosemide was detected using a UV absorbance detector, whereas amiloride, because of its very low therapeutic range, was detected with a spectrofluorimetric detector. The linearity of the furosemide and amiloride assays were confirmed over the range of 30-3000 ng/ml and 0.5-30 ng/ml, respectively. These concentrations correspond well with the therapeutic ranges of both drugs. The extraction recoveries, depending on concentration, exceed 80% for furosemide and 74% for amiloride. The reported methods were applied to pharmacokinetic investigations of the two compounds taken in form of a drug combination.

Benefits and risks of torasemide in congestive heart failure and essential hypertension

Torasemide is a lipophilic anilinopyridine sulphonylurea derivative that acts as a high ceiling loop diuretic and has been used for the treatment of both acute and chronic congestive heart failure (CHF) and hypertension. Torasemide is similar to other loop diuretics in terms of its mechanism of diuretic action; namely, blockade of Na+/K+/2Cl- cotransport in the thick ascending limb of the loop of Henle. It has high bioavailability (> 80%), as does bumetanide, but a longer elimination half-life (3 to 4 hours) than either bumetanide or furosemide (frusemide). In the treatment of chronic CHF, oral torasemide (5 to 20 mg/day) has been shown to be an effective diuretic. Patients treated with torasemide for up to 1 year have reduced bodyweight, improved pulmonary haemodynamics, and decreased CHF severity. Intravenous torasemide (20 to 60mg as a single dose) has been shown to be as effective as furosemide in the treatment of acute CHF, and resulted in significant diuresis, bodyweight loss, and improved pulmonary haemodynamics and exercise performance. 'Non-diuretic' dosages (2.5 to 5 mg/day) of oral torasemide have been used to treat essential hypertension, both as monotherapy and in combination with other antihypertensive agents. When used in these dosages, torasemide lowered diastolic blood pressure (DBP) to below 90mm Hg in 8 to 12 weeks in 70 to 80% of patients. With dose doubling, this level of efficacy occurred in more than 90% of hypertensive patients. Clinical trials have established that blood pressure can be maintained at this level for at least 1 year with low dose torasemide. Torasemide is well tolerated in dosages up to 20 mg/day for at least 1 year. The most commonly reported adverse effects are those associated with loop diuretics in general. These include transient hypokalaemia, hyperuricaemia, dizziness, headache, gastrointestinal disturbances, orthostatic hypotension and fatigue. Adverse effects are comparable with those of other diuretics and rarely necessitate drug withdrawal.

Decrease in the time-dependent difference in urinary excretion of furosemide with age

The influence of age on the time-dependent difference in urinary excretion of furosemide, a loop diuretic agent, was examined in this longitudinal study. Male Wistar rats were maintained under conditions of light from 07:00 to 19:00 h and dark from 19:00 to 07:00 h. Furosemide (30 mg/kg) was given orally at 12:00 h (day trial) or 00:00 h (night trial) to rats at 3 months of age, and urine was collected for 8 h after dosage. Thereafter, the identical protocol was repeated using the same animals at 6, 9, 12, 15, 18, and 21 months of age. The urinary excretion of furosemide was significantly greater in the day than in the night trial at 3 months of age. Such a time-dependent difference was observed for up to 15 months, but disappeared at 18 and 21 months of age. The time-dependent difference in urinary excretion of furosemide (day trial - night trial) decreased gradually throughout the observation period of the study. These results suggest that the time-dependent difference in the urinary excretion of furosemide diminishes during the aging process and disappears by 18 months of age in male Wistar rats.

Daily variation in the urinary excretion of furosemide in young and aged rats

We have recently demonstrated that the time-dependent difference in urinary excretion of furosemide, a loop diuretic, diminishes during the aging process and disappears by 18 months of age in rats. The present study was undertaken to examine whether the amplitude of the daily variations in the urinary excretion of furosemide or their pattern, or both, are influenced in aged animals. Young (3 months of age) and aged (30 months of age) Wistar rats were maintained under conditions of light from 7 am to 7 pm and dark from 7 pm to 7 am. Furosemide (30 mg/kg) was given orally at 4 am, 8 am, 12 am, 4 pm, 8 pm or 12 pm. Urine was collected for 8 hours after furosemide administration and urinary excretion of furosemide was determined. There were significant daily variations in the urinary furosemide and the urine volume with the peak at 8 am and the trough at 12 pm in both groups of rats. The differences in these parameters between the 8 am and 12 pm trials were significantly smaller in the aged than in the young rats. These results suggest that the age-related alteration in the time-dependent phenomenon of furosemide is caused by the decreased amplitude of the daily variation in the urinary furosemide excretion and its diuretic effect.

Physiological changes due to age. Implications for cardiovascular drug therapy

Cardiovascular disease is the single largest cause of death in the elderly. Many of the published studies concerning the physiology and pharmacology of the aging cardiovascular system are seriously flawed. Problems include failure to measure the drug bioavailability and the selection of subjects with overt or subclinical disease. With exercise, the rise in heart rate is inversely proportional to age and maximum heart rate is reduced. Baroreceptor reflex activity appears to decline with age. Cardiac output is maintained in the elderly, with a slower heart rate and a greater stroke volume than in the young. Plasma noradrenaline (norepinephrine) levels increase in the elderly but there is no change in the sensitivity of the vasoconstrictor alpha 1-adrenoceptor. There is evidence for a decline in the activity of the vasodilator beta 2-adrenoceptor with age. It is difficult to make general rules about the effect of aging on the disposition and elimination of drugs. Each drug must be tested separately.

Multiple linear regression modeling of furosemide renal clearance and urinary excretion rate

Multiple linear regression techniques were utilized to determine models for the renal clearance and urinary excretion rate of furosemide. Models for the renal clearance were formulated based on data collected from the literature. The best model predicted that the weight-normalized renal clearance was a function of the weight-normalized creatinine clearance, with coefficient values dependent on the presence or absence of heart, liver, and/or kidney failure. The predictive performance of this model was evaluated using a separate verification data set, and, prospectively, for a group of cardiac patients. The urinary excretion rate of furosemide is the primary determinate of response. Models for the furosemide excretion rate were formulated from data collected prospectively from a group of patients with cardiac disease. The best model predicted that the dose-normalized morning urinary excretion rate was a function of the blood urea nitrogen concentration (BUN), with modifications for the presence of liver failure and/or decompensated heart failure. The oral dosage required to produce a clinically optimal furosemide excretion rate in cardiac patients without liver disease was dose (mg) = 42.1/(0.925-0.0151 BUN).

Furosemide (frusemide). A pharmacokinetic/pharmacodynamic review (Part II)

Part I of this article, which appeared in the previous issue of the Journal, covered the physical properties, pharmacology, toxicology and pharmacokinetics of furosemide (frusemide). In part II the authors examine the pharmacodynamics of the drug, and suggest various areas for future study.

Effects of aging on responses to furosemide

Furosemide causes both diuretic and non-diuretic changes in renal function. We compared responses to intravenous furosemide 0.5 mg.kg-1 in 38 subjects (30 males, 8 females) aged 18 to 30 with those in 14 subjects (9 males, 5 females) aged 50 and over. There were no consistent differences attributable to gender. Older persons showed greater natriuresis (47 percent in males and 26 percent in females) but their increment in plasma renin activity was markedly reduced. The urinary excretion of thromboxane B2 was elevated in older subjects (58 +/- 10 vs. 30 +/- 4 ng/4 h, p less than 0.05 for males; 48 +/- 7 vs. 29 +/- 4 ng/4 hr, p less than 0.05 for females) while that of 6-keto prostaglandin F1 alpha was not different. While differences in the diuretic response to furosemide may be due to pharmacokinetic differences, the non-diuretic response differences may reflect age related changes in renal prostaglandin synthesis.

Furosemide pharmacokinetics and pharmacodynamics in health and disease--an update

The literature on furosemide pharmacokinetics and pharmacodynamics is critically reviewed, concentrating on those papers published subsequent to the 1979 reviews of this topic. Intravenous and oral data are presented for healthy volunteers and for patients with various disease states. It is the latter populations about which the majority of the studies have been published since 1979. Inter- and intraindividual variations in bioavailability are discussed, as are data on the metabolism of furosemide to its glucuronide conjugate. Published studies examining the relationship between furosemide pharmacodynamics and pharmacokinetics are also evaluated. The literature is reviewed through June 1988.

The effect of furosemide on vascular smooth muscle is influenced by plasma protein

The effect of furosemide on vascular smooth muscle was studied on segments of rabbit blood vessels. Furosemide (20 micrograms/ml) induced small (3-4%) decreases in resting tension whereas nitroglycerin (0.5 micrograms/ml) induced decreases from 8 to 13%. The addition of albumin (45 g/l) abolished the furosemide response whereas the nitroglycerin response was unaffected. Similarly the pronounced effect of nitroglycerin on electrically evoked contractions was unaffected by the presence of albumin whereas the furosemide effect was weakened by albumin. The biologically determined free fraction of furosemide increased with increasing drug concentrations from 13% at 16 micrograms/ml to 35% at 256 micrograms/ml.

Clinical pharmacokinetic considerations in the treatment of increased intracranial pressure

Life-threatening increased intracranial pressure can be reversed by a variety of drugs. These compounds all have some disadvantages, producing rebound effects, severe coma or cardiovascular depression and electrolyte imbalance. However, reduction of intracranial pressure is a prerequisite for recovery and the benefits of treatment outweigh the risks. Dexamethasone is rapidly eliminated, the short half-life (about 3 hours) indicating that dosage intervals should be kept small. As yet, however, its therapeutic efficacy has not been clearly demonstrated. Therefore, an association between pharmacokinetics and pharmacodynamics cannot be established. Osmotic diuretics are the most widely used agents for reduction of intracranial pressure. Pharmacokinetics show a very close relationship to changes in serum osmolality, but there are large variations in the clearance. For the use of osmotics, the blood-brain barrier must be intact. Osmotic diuretics may lead to intracerebral oedema or to acute renal failure as serum osmolality increases. Considering the pharmacokinetics of each drug, and the dynamics of intracerebral pressure and osmolality, an intermittent, individually titrated dosage should be administered, with simultaneous monitoring of intracranial pressure. Frusemide (furosemide) can be used as an adjunct, to enhance the effect of osmotic diuretics. Its pharmacokinetics are limited by renal function, depending on age as well as on the extent of renal impairment. Altered renal elimination of concomitantly administered drugs, and electrolyte imbalances should be anticipated when diuretics are used. Barbiturates are certain to decrease intracranial pressure in humans by an as yet unknown mechanism. Their administration is recommended for patients that do not respond to conventional therapy. As barbiturates can result in deep coma, knowledge of their pharmacokinetics is of great importance for recovery. Following single doses, pentobarbitone has a relatively long elimination half-life (about 22 hours). However, after repeated administration for several days, its elimination may be enhanced due to autoinduction. Thiopentone kinetics are characterised by distribution and redistribution into deep peripheral compartments. Administration of high and frequent doses leads to considerably delayed recovery. This is not true for methohexitone, which shows comparable pharmacokinetics after single and multiple dose administration. Elimination depends on liver blood flow. Thus, recovery from methohexitone-coma is rapid. Rapid elimination is also an important characteristic of etomidate and alphaxalone/alphadolone, two non-barbiturate hypnotics.(ABSTRACT TRUNCATED AT 400 WORDS)

Plasma protein binding of drugs in the elderly

Binding to plasma proteins can affect the pharmacokinetics and pharmacodynamics of drugs. Age is one of many factors which can affect plasma protein binding of drugs. Unfortunately, very few generalities can be drawn from the studies of the effect of age on protein binding. Whether age has an effect on protein binding is dependent not only on the drug, but also on the manner in which the study is conducted. Several studies involve patients with various disease states making assessment of the effect of age alone on protein binding difficult. Results of different studies on the same drug do not always agree--in one case finding no change in protein binding with age and in another, a significant increase or decrease in protein binding. Most drugs which exhibit increased binding (decreased free fraction) in elderly subjects are basic and tend to have a greater affinity for alpha 1-acid glycoprotein than for albumin. The list of drugs exhibiting decreased binding (increased free fraction) in the elderly is longer and includes both acidic and basic drugs. The impact of changes in protein binding with age is dependent on the magnitude of the change, on the pharmacokinetic characteristics of the drug and on its therapeutic index. Some changes, although statistically significant, are not likely to be of importance clinically. From the studies reviewed, the free fraction is changed by greater than 50% in the elderly for only a few drugs, e.g. acetazolamide, diflunisal, etomidate, naproxen, salicylate, valproate and zimeldine.

Disposition and response to bumetanide and furosemide

Bumetanide and furosemide are potent loop diuretics; the former is 40 to 50 times more potent than the latter on a weight basis. Bumetanide is absorbed more quickly than furosemide and is twice as bioavailable. Both drugs exhibit changes in elimination in the presence of renal insufficiency as well as changes in the time course of absorption in congestive heart failure. More data are needed to assess potential differences between them in various clinical conditions.

Diuretic drugs. Progress in clinical pharmacology

Oral diuretics are amongst the most widely used drugs in clinical practice today. Their discovery close on thirty years ago remains a major milestone in therapeutic progress. Though originally designed for treating heart failure, diuretics are more commonly prescribed, worldwide, in hypertension than for relief of oedema. Since the introduction of chlorothiazide, diuretic development has passed through a series of distinct stages. The thiazide era was followed by the 'high-ceiling' diuretics, the antikaliuretics and, more recently, polyvalent agents that cause both saluresis and uricosuria. Alongside these synthetic achievements, major advances have occurred in the knowledge of nephron function and ion transport mechanisms. These have acted as stimulus to the design of novel categories of diuretics. The practising clinician thus has a wide range of available diuretics to choose from. The most appropriate choice of an agent aimed at the relief of symptoms with minimal adverse effects requires an understanding of where and how diuretics act within their primary target organ, the kidney. Whereas various procedures, ranging from micropuncture to the study of brush border membrane vesicles, have been utilised experimentally, investigation of the mode and sites of action of diuretics in man has largely depended on application of clearance methodology. Refinements in analytical chemistry have encouraged study of the pharmacokinetic and metabolic fate of diuretics. Taken together, available evidence shows that most diuretics exert their saluretic action from the intraluminal aspect of the renal tubules. The time-course of drug delivery, as well as total quantity of drug transported into the lumen determine the cumulative drug response. Exceptions are muzolimine and the aldosterone antagonists which act at the peritubular membrane. Distinctive stereospecific effects on luminal tubular ion transport occur with indacrinone and etozoline. The clinical use of diuretics often involves concurrent administration with other drugs. The mechanisms involved in a number of the resulting pharmacodynamic and pharmacokinetic interactions have considerable relevance in patient management. Notable examples of these interactions are the blunting of diuretic action by non-steroidal anti-inflammatory agents and the diuretic-induced diminution in the renal clearance of lithium salts.

The elderly patient. A special case for diuretic therapy

Diuretics are among the most widely prescribed drugs, especially for the elderly with cardiac failure or hypertension. Progressive structural and functional changes occur in the kidneys after the fourth decade, leading to impairment of the ability of the kidneys to handle sodium, water and solutes. The renal reserves of the elderly are about half those of the young. In addition, the renin-aldosterone system shows reduced activity in old age. The pharmacokinetics and pharmacodynamics of diuretics in the elderly are reviewed, and the influence of congestive cardiac failure is emphasised with regard to the kinetics of diuretics and the deleterious effect of diuretic-induced hypokalaemia and hypomagnesaemia on the pharmacology of digoxin. Guidelines are suggested for the use of diuretics in the elderly, including the avoidance of unnecessary use, the careful choice of diuretic used, the need for small initial doses, and the prevention of hypokalaemia. The place of potassium-sparing agents for the elderly and adverse effects of diuretics, either mechanical, metabolic or toxic are discussed. Mechanical problems are related to the rate and volume of urine produced, and the resulting effects on bladder function and on blood volume. Although toxic effects are relatively rare, metabolic effects include electrolyte changes, impairment of glucose tolerance, and increased serum uric acid and lipids. Most of these adverse effects are preventable by careful management; the consensus is that they are not of sufficient clinical significance to outweigh the long record of efficacy and safety of diuretic therapy in the elderly. Diuretics will, and should, continue to be used extensively in elderly patients with hypertension and/or cardiac failure.

Resistance to loop diuretics. Why it happens and what to do about it

Resistance to loop diuretics is often encountered clinically. Studies in healthy subjects have shown that overall response to loop diuretics depends upon the interplay between the total amount of drug reaching the urine, the time course of its entry into urine and the pharmacodynamics of response to diuretic in the urine. The mechanism by which diuretic resistance occurs has been elucidated in several clinical conditions. Treatment with inhibitors of prostaglandin synthesis has no effect on diuretic appearance in urine but blunts response by blocking the increase in renal blood flow produced by loop diuretics. In the elderly and in patients with moderate renal insufficiency, the mechanism of resistance appears to be purely pharmacokinetic, involving altered access of diuretic into the urine. In contrast, patients with nephrotic syndrome and hepatic cirrhosis manifest a purely pharmacodynamic form of resistance: in nephrosis, diuretic may bind to protein in the urine; in cirrhosis the mechanism of resistance is unclear. Lastly, in patients with congestive heart failure, with intravenous administration, resistance represents a pharmacodynamic phenomenon. With oral administration, however, the time course but not the extent of absorption is altered; consequently, in this setting, both pharmacokinetic and pharmacodynamic changes may contribute to the subnormal response. Strategies for overcoming resistance to loop diuretics in patients receiving NSAIDs or those with renal disease, hepatic cirrhosis or congestive heart failure include one or more of: increasing the dose size; administering frequent 'small' (but effective) doses; continuous intravenous infusion of the diuretic; or concomitant administration of another diuretic such as metolazone or hydrochlorothiazide.

Diuretics. Clinical pharmacology and therapeutic use (Part I)

25 years have elapsed since the introduction of the first effective oral diuretic, chlorothiazide. Diuretics are now amongst the most widely prescribed drugs in clinical practice worldwide. Availability of these drugs has not only brought therapeutic benefit to countless numbers of patients but it has at the same time provided valuable research tools with which to investigate the functional behaviour of the kidney and other electrolyte-transporting tissues. Despite many remaining gaps in our knowledge of the biochemical processes involved in diuretic drug action, available compounds can be divided into 5 groups on the basis of their preferential effects on different segments of the nephron involved in tubular reabsorption of sodium chloride and water. Firstly, there is heterogeneous group of chemicals that share the common property of powerful, short-lived diuretic effects that are complete within 4 to 6 hours. These agents act on the thick ascending limb of Henle's loop and are known as 'high ceiling' or 'loop' diuretics. The second group are the benzothiadiazines and their many related heterocyclic variants, all of which localise their effects to the early portion of the distal tubule. The third group comprises the potassium-sparing diuretics which act exclusively on the Na+-K+/H+ exchange mechanisms in the late distal tubule and cortical collecting duct. The action of drugs in groups 2 and 3 is prolonged to between 12 and 24 hours. The fourth group consists of diuretics that are chemically related to ethacrynic acid but have the unusual property of combining within the same molecule the property of saluresis and uricosuria. These compounds have actions, to different individual extents, in the proximal tubule, thick ascending limb, and early distal tubule and are known as 'polyvalent' diuretics. Finally, there is a mixed group of weak or adjunctive diuretics which includes the vasodilator xanthines such as aminophylline, and the osmotically active compounds such as mannitol. Available evidence on the molecular mechanisms of action of diuretics in each group is reviewed. The haemodynamic, humoral and physical factors involved in control of electrolyte and fluid handling by the kidney in normal conditions and pathological states are discussed in relation to rational choices of different diuretics in the treatment of various oedematous and non-oedematous conditions.

Bioavailability and diuretic effect of furosemide during long-term treatment of chronic respiratory failure

The bioavailability and diuretic effect of furosemide 40 mg administered orally for at least 6 months have been compared in patients with chronic respiratory failure and in healthy controls. The mean urinary recovery of unchanged drug was 11.5 mg and 9.41 mg in 24 h after pre- and postprandial administration to 10 patients, whereas the recovery was 14.4 mg in 10 healthy subjects. The diuretic effect, in terms of urine flow and sodium ion excretion in the 6 h after administration, was also less in patients than in healthy subjects. This was ascribed to the lower bioavailability of furosemide in patients, based on the urinary recovery of unchanged drug, and not to a lower level of response to furosemide than in healthy subjects. The mean absolute bioavailability of furosemide in 6 patients was 41.3% and 63.4%, respectively, calculated from unchanged drug and total drug (unchanged plus glucuronide conjugate). Approximately 53.9% of the dose of furosemide was excreted as the glucuronide conjugate after oral administration, and 34.2% after i.v. injection in the 6 patients. In 3 of the 6 patients studied, a distinct first-pass effect for glucuronidation of furosemide was observed after oral administration. In another study, the mean glucuronide fraction recovered in 24-h urine was 20.7% and 7.3% (p less than 0.01) in 38 patients and 12 healthy subjects, respectively. The fraction in urine was not affected by changing the dose of furosemide from 20 to 120 mg. The lower bioavailability in patients as compared to healthy subjects is ascribed to enhanced glucuronidation and incomplete drug absorption.

The influence of age on renal and extrarenal effects of frusemide

The effect of frusemide 80 mg i.v. was compared during 24 h in 10 young and eight elderly healthy male volunteers following a 24 h control period in the ward. During the 30 min following the injection the increments in excretion of urine, sodium, potassium and frusemide were significantly smaller in the elderly. The 24 h increase in sodium excretion was significantly larger in the 0 young and eight elderly healthy male volunteers following a 24 h control period in the ward. During the 30 min following the injection the increments in excretion of urine, sodium, potassium and frusemide were significantly smaller in the elderly. The 24 h increase in sodium excretion was significantly larger in the 0 young and eight elderly healthy male volunteers following a 24 h control period in the ward. During the 30 min following the injection the increments in excretion of urine, sodium, potassium and frusemide were significantly smaller in the elderly. The 24 h increase in sodium excretion was significantly larger in the elderly. The endogenous 24 h creatinine clearance was reduced by 12% (P less than 0.01) in both age groups. The frusemide induced changes in the 8 h serum concentration curves for albumin differed significantly between the two groups (analysis of variance, P less than 0.01). The drug induced increase in albumin concentration became significant in the young 5 min after the injection. In the elderly it took more than 15 min before the increase in serum albumin reached significance. The average maximal increase in albumin concentration was 14.3% in the young and 9.7% in the elderly (P less than 0.05). No difference was seen between the two age groups in the significant frusemide induced increases in the 24 h albumin excretion but in the elderly a significantly larger decrease in the 24 h excretion of beta 2-microglobulin was observed (P less than 0.05). No significant age difference was observed in the initial significant increases in diastolic blood pressure observed in both age groups or between the later changes in systolic blood pressure which was significantly reduced in the young only. The slower haemoconcentration response in the elderly seemed associated with the slower secretion rate of frusemide to the tubular lumen. We found no evidence of an age related difference in tubular cell response to frusemide. It is emphasized that a maximal initial frusemide response in the elderly, in contrast to what was found in the young, probably was not achieved by the 80 mg i.v.

The influence of intravenous furosemide on the renal excretion pattern of protein and protein degradation products

To assist the attending physician who may have difficulties in deciding whether observed deviations from normal renal physiology are disease-induced or diuresis-induced we studied urinary excretion rate of proteins and protein degradation products in eight healthy male volunteers between 60 and 70 years. After a 24 hr control period in the ward the subjects received 80 mg furosemide intravenously. The 24 hr creatinine clearance was reduced from 95 +/- 9 to 83 +/- 6 ml/min. (P less than 0.05). The reduction in the 24 hr carbamide clearance from 35.8 +/- 9.4 to 33.1 +/- 6.2 ml/min. was not significant. The urinary pH was significantly increased during the first hour but after the pH showed a fall which was significant during 2-8 hours after the injection. Initially the urate excretion showed an apparent increase but from 1-8 hours after the injection significantly less urate was excreted. The 24 hr urate clearance was reduced by furosemide from 5.3 +/- 1.6 to 3.4 +/- 1.1 ml/min. (P less than 0.05) and the amount of urate excreted was reduced from 2.6 to 1.6 mmol per 24 hrs (p = 0.05). The excretion rate of beta 2-microglobulin was reduced (P less than 0.01) whereas the excretion rate of albumin was increased (P less than 0.05).